Piston-cylinder like machines are used in industry for the compression of certain gases to very high pressures. These gases at very high pressures have various uses, in particular, in the manufacture of polyethylene plastics.
One of the intermediate steps in the manufacture of these plastics is the compression of the polyethylene gas to pressures of approximately 40,000 p.s.i. The compression of the polyethylene gas may take place in several stages using piston-cylinder type apparatus to achieve the desired final pressure of the polyethylene gas.
Because of the very high pressures involved with the compression of the polyethylene gas, the piston-cylinder like machines usually consist of heavily reinforced cylinders having a compression chamber therein, the compression chamber having a high length-over-diameter ratio thereby requiring a cooperating plunger which, also, must have a very high length-over-diameter ratio. Seal means are usually provided between the compression cylinder and the plunger so that the plunger may reciprocate longitudinally in the compression cylinder.
With the piston in its first position in the heavily reinforced cylinder, inlet gases are fed into the compression chamber through an inlet means into the chamber. The valve in the inlet means is then closed and the piston is advanced into the compression chamber compressing the gas in the compression chamber and forcing the gas to flow through an outlet means into a further compression chamber at a higher pressure.
The outlet means is normally provided with a check valve or other equivalent to prevent the higher pressure gases from the second compression chamber from backflowing into the compression cylinder when the plunger is retracted outwardly from the compression chamber to allow the compression chamber to accept more gas from the inlet means.
The very high pressures created in the compression chamber and forces on the plunger required to create those pressures in the above described apparatus require that the outside diameter of the plunger and the inside diameter of the compression cylinder form a very close fit and yet still have a smooth sliding engagement to allow the plunger to reciprocate in the compression cylinder. As the plunger is advanced and begins to compress the gases, extremely high forces on the plunger and the high length-over-diameter ratio of the plunger require that the plunger be extremely rigid so as to maintain a very strict overall straightness requirement.
Straightness is required to reduce unnecessary wear on the plunger and seals while engaging with the cylinder and to further prevent the unsupported length of the plunger from fracture. Further, the outside diameter of the plunger must be made of a hard wear resistant material in order to reduce wear on the plunger and seals during its sliding engagement with the compression cylinder walls and must further resist undue radial expansion when encountering compressive stresses.
Previously, machines as described above have used solid plungers formed of a hard wear resistant material; this material has operated very satisfactorily in these types of apparatus. The extreme rigidity provided by a hard wear resistant material and its superior strength when in compression have helped to reduce any catastrophic failures which can and do occur when the plunger is advanced into the compression cylinder.
These catastrophic failures usually occur because there is a long, unsupported length of the plunger when it first begins to advance into the compression cylinder and compress the gases. Plungers formed of material such as cemented hard metal carbide have been found very satisfactory for plungers up to a diameter of approximately 3 to 4 inches and an overall length of approximately 40 to 50 inches.
Recently, however, piston-cylinder like apparatuses as described above have been made in which the diameter of the chamber used to compress the gases has been increased to approximately six or seven inches. The cemented hard metal carbide plungers which were used with the smaller diameter compression chambers normally had a weight of approximately 400 pounds, but to make a solid cemented hard metal carbide plunger to cooperate with the newly enlarged compression chambers would require a plunger whose weight might exceed 1,000 pounds and become very cumbersome to manufacture and handle. Furthermore, such massive carbide members are difficult to sinter without developing cracks or fissures therein or setting up strains therein.
In addition, hard wear resistant materials such as cemented hard metal carbide are very expensive materials as compared to steel and, therefore, the economic efficiency of producing such a plunger becomes questionable.
It is an object of the present invention to provide the enlarged plungers without making the plungers of a solid piece of carbide and yet to maintain the structural integrity necessary for the plunger to operate in such extreme environments.
It is a further object of the present invention to produce economically feasible plungers for use in the enlarged compression cylinders.